Dual rate sear mechanism



Aug. 15, 1961 F. P. REED 2,995,988

DUAL RATE SEAR MECHANISM Filed Nov. 2, 1960 4 Sheets-Sheet 1 INVENTOR. i Fm; dETickPHEE' Ii BY Aug. 15, 1961 F. P. REED 2, 95,988

DUAL RATE SEAR MECHANISM Filed Nov. 2, 1960 4 Sheets-Sheet 2 Fig.5-

IN V EN TOR. .FTE [121 1 nkppe Ed 12 QOMVQJ.

Aug. 15, 1961 F. P. REED DUAL RATE SEAR MECHANISM 4 Sheets-Sheet 3 Filed Nov. 2, 1960 INVENTOR. Frederick nee :1

Aug. 15, 1961 F. P. REED DUAL RATE SEAR MECHANISM 4 Sheets-Sheet 4 Filed NOV. 2, 1960 IN VEN TOR. Frederick p.11 EE :1

United States Patent F 2,995,988 DUAL RATE SEAR MECHANISM Frederick P. Reed, Ludlow, Mass., assign'or to the United States of America as represented by the Secretary of the Army Filed Nov. 2, 1960, Ser. No. 66,890 6 Claims. (Cl. 89-129) (Granted under Tifle 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment of any royalty thereon.

This invention relates to the sear mechanisms of firearms and more particularly to sear mechanisms for effecting a dual rate of fire.

One of the most dangerous hazards encountered in the use of machine guns is a run-away condition where firing cannot be stopped because the sear, when released by the trigger, does not positively engage the bolt. Runaway conditions are particularly prevalent in those machine guns where the sear is pivotally disposed because, in such arrangement, the impact forces created when the sear engages the counterrecoiling bolt are easily converted to forces which cause the sear to pivot for release of the bolt. The general practice for overcoming the hazard is to incline the engaging surfaces of the sear and bolt so as to direct the impact forces relative to the axis of the sear so that the sear will be pivoted by the forces in the direction of engagement. This, however, means that the sear, when pivoted to release the bolt, must move the bolt against its actuating spring and thereby create a heavy trigger action. To reduce this disadvantage as much as possible, the engaging surfaces on the bolt and sear are inclined to direct the impact forces as close to the axis thereof as is possible for a lighter trigger action. But as a result, the tendency of the sear to make insecure engagement with the bolt is increased.

Moreover, there are those machine guns in which a reduced rate of fire is selectively achieved by pivotally mountingthe scar on a member which is spring-biased to resiliently oppose the counter-recoil movement imparted to the member by the bolt when engaged by the sear. In such a sear mechanism, an actuator is disposed so as to be selectively positioned for camming engagement by the sear during forward displacement thereof by the engaged bolt to pivot the sear for release of the bolt.

- With such a sear mechanism, the bolt has a tendency to vibrate between the sear and the recoil buffer when the sear is released by the trigger to terminate a firing burst with the vibrations causing the sear to be pivotally displaced to release the bolt.

It is, therefore, one object of this invention to provide a dual rate sear mechanism wherein there is provided a primary sear which is mounted for sliding displacement normal to the longitudinal displacement of the bolt J so that the impact forces cannot be converted to forces which will disengage the sear from the bolt.

It is another object of this invention to provide such a' sear mechanism having a secondary sear mounted on a scar block which is mounted forlongitudinal displacement on a trigger block which carries the slidingly mounted primary sear, and mechanical means are pro- Patented Aug. 15, 1961 dual rate sear mechanism which is positive in operation, simple in design and easy to manufacture. I

The specific nature of the invention as well as other objects and advantages thereof will clearly appear from a description of a preferred embodiment as shown in the accompanying drawings in which:

FIG. 1 is a longitudinally cross-sectioned view of the sear mechanism showing relationship of the parts when selected for full rate fire and the trigger bar is actuated to firing position;

FIG. 2 is a view taken along line 2-2 of FIG. 1;

FIG. 3 is a longitudinally cross-sectioned view showing the bolt engaged by the primary sear;

FIG. 4 is a view similar to FIG. 3 but showing the primary sear actuated to release the bolt and the secondary sear actuated for engagement therewith;

FIG. 5 is a view taken along line 5-5 of FIG. 3;

FIG. 6 is a view taken along line 66 of FIG. 4;

FIG. 7 is a view taken along line 77 of FIG. 5;

FIG. 8 is an exploded perspective view of the sear block assembly; and

FIG. 9 is an exploded perspective view of the trigger block assembly.

Shown in the figures is a firearm 12 which fires from an open bolt position and which is provided with a receiver 13 in which a bolt 14 is slidingly disposed for reciprocation between a forward battery position and a rearward recoil position. The positions and directions of displacement of the structures to be defined hereinafter will be given respective to the axes of receiver 13 and looking towards the front end thereof from the rear.

A buffer device16 is provided in the rear end of receiver 13 to resiliently stop bolt 14 when in the recoil position and a. drive spring 18 is disposed so as to be compressed during recoil of the bolt for re-energizing the bolt for counterrecoil movement to the battery position. A trigger block 20 is fixedly mounted to receiver 13 adja cent the rear end thereof and such trigger block slidingly supports a sear block 22 for longitudinal movement respective thereto. Trigger block 20 and sear block 22 extend through receiver 13 for cooperation with bolt 14 by means of an opening 21 and the sear block is arranged to reciprocate a predetermined distance x-x between a forward contact with the trigger block and a rearward contact with the rear end of opening 21.

Sear block 22 includes a housing portion 23 which extends radially through opening 21 and a cylinder 24 which extends rearwardly from the outer end of the housing portion. Cylinder 24 is provided with an axial bore 26 which extends rearwardly from the front end and is counterbored from the rear end, as noted at 218, to form an annular shoulder 30. Cylinder 24 carries a piston 32 having a diameter similar to that ofcounterbore 28 so as to be slidingly received thereby and a stem 34 having the same diameter as bore 26 extends from the piston for extension through cylinder 24 so that the end of stem 34 contacts a radially disposed wall 36 on trigger block 20. A compressible coil spring 38 is housed in counterbore 28 rearwardly of piston 32 and a nut 39 is threadingly mounted in the rear end of the counterbore so as to preload the spring. The front end of piston '32 contacts the rear end of trigger block 20 whereby, 'with the trigger block being fixed to receiver 13, the forward displacement of sear block 22 respective to the t-iigger block is resiliently resisted by spring 38 which re- I siliently returns the sear block to its normal rearward position in contact with the rear end of opening 21.

A sear 40 with a cylindrical stem and an'essentially rectangular latch portion 42 is radially disposed ina mat+ ting hole in sear block 22 and is spring-biased inwardly by a spring 41 so that the latch portion is engageable It is still another object of this invention to provide a I with a cooperating locking lug 44 on bolt 14 for seat- 3 ing the bolt in the recoil position thereof. Cooperating ramps 46 and. 47 are provided, respectively, on. latch portion 42 and locking lug 44 so that sear 40 is cammed upwardly out of the way of the locking lug during recoil movement of bolt 14 to permit contact of the front end of the locking lug with the latch portion for holding the bolt in the recoil position.

Provided in sear '40 is a longitudinal aperture 48 which communicates with an opening 49 extending through housing portion 23. to the front end thereof. A pin 50 extends laterally through opening 49 and pivotally mounted on such pin is a lever 52 having a sear engag ins. end 58 which extends into aperture 48 and a forward end 54 which projects forwardly to extend into trigger block20. Whereby, pivotal displacement of lever 52 is converted to translational displacement of sear 40 for actuation between a bolt holding position and a bolt release position. Forward end 54 is provided with a. planar surface 55 along the top thereof and such planar surface is arranged so as to be longitudinally disposed when sear 40 is displaced by lever 52 to the bolt holding position.

Provided in sear block 22 below aperture 48 is a cylindrical; hole 56 which extends longitudinally into sear block 22 from the front face thereof. Hole 56 receives the rear end of a cylindrical actuator 57 so as to extend longitudinally forward from the front end of sear block and the actuator is secured against displacement in the hole by means of a transversal pin 58. The portion of actuator 57 which extends forwardly from sear block 22 is slabbed along the underside to form a recess 59, which is terminated at the front end of a radial wall 60 and a. front end 61.

A trigger bar 62 is pivotally mounted on trigger block 20 by means of a longitudinally disposed pin 64 which extends through the trigger block adjacent the top right corner thereof. Whereby, one end 66 of trigger bar 62 extends across the top of trigger block 20 while the other end forms a trigger 68 which extends laterally outward therefrom. An armed coil spring 70 is mounted over pin .64 and is arranged so as to bias end 66 away from the top of trigger block 20. A lug 72 on trigger bar 62 is. oontactable with the right side of trigger block 20 to=limit the displacement of the trigger bar by spring 70.

A recess 73 extends into end 66 of trigger bar '62 from the end thereof and such recess receives a trigger release 74. which is disposed for rotation in a lateral plane around a pin 76 disposed longitudinally through the recess. Trigger release includes a depending leg 78 and a lip 80 located on the left side thereof. A spring-biased pin 84 is housed in. trigger bar 62' above pin '64 so as to press against trigger release 74 above pin 76 whereby the trigger release is biased for counterclockwise rotation with such rotary displacement being limited by the contact of a; flat '82 on the. trigger release. with the end of recess 73. With trigger release 74 in the normal position thereof, wherein flat 82 is in contact with the end of recess 73, the end of leg 78. is contactable. with the top of a cylin. drical plunger 86 which is mounted in trigger block 20 for sliding vertical displacement. Plunger 86. is so disposed in trigger block 20 that the. bottom end is contactable with planar surface 55. on forward end 54 of lever 52'. whereby, when trigger -68 of trigger bar '62 is pulled upwardly, as by lanyard means, the resulting downward disp acement of end 66 thereof is transferred through trigger release 74 to plunger 86 and therefrom through lever 52 to. sear 40 which is actuated to the bolt release position.

Extending through trigger block 20,. on the right side of: leg- 78', is a pin 87 which pivotally supports a disconncctor '88. Disconnector 88 is substantially of inverted L-configuration and includes an arm '90, which extends to the right from pin 87', and a leg 92 which extends downwardly through a vertical crevice 94 in trigger block 20. A spring-biased plunger 96 is vertically disposed in trigger bloek 2 to the right of crevice 94 and is arranged to press against the underside of arm so as to be biased upwardly and consequently Pivo g 2 out of vertical disposition to the right until the end thereof contacts the right wall of the crevice. The top side of arm 90 is contactable by a nub 97 so that, when trigger 68 is pulled upwardly, disconnector 88 is swung in a clockwise direction to position leg 92 vertically. Dis connector 88 is positioned so that the lower portion of leg 92 is engaged by the front end of lever 52 during forward displacement of sear block 22 relative to trigger block 20. The forward displacement of scar block 22 is converted to pivotal displacement of lever 52, to actuate forward end 54 thereof downwardly, by means of cooperating cam surfaces 98 and 100 provided respectively on the lower portion of leg 92 and the front portion of the forward end. The pivotal displacement of lever 52 is suff cient to actuate sear 40 from the bolt holding to the bolt release position.

A selector 102' is pivotally mounted on the left side of trigger block 20 by means of a longitudinally disposed pin 104 which traverses the selector approximately midway the top and bottom ends thereof. Selector 102 is Positioned adjacent lip 80 so that, when the top portion of the selector is pressed inwardly, the top end of the selector is positioned under the lip for contact thereby when trigger release 74 is displaced downwardly by the upward pull against trigger 68. Thus, when selector 102 is actuated to displace the upper end thereof inwardly and trigger 68 is pulled upwardly, the contact of the upper end of the selector by lip 80 causes trigger release 74 to rotate and thereby swing leg 78 olf of plunger 86 after the plunger is pushed downwardly against lever 52 to actuate. sear 40. from the bolt holding to the bolt release position. At the same time, nub 97 presses down against. arm 90 of disconnector 88 to swing leg 92 to its vertical position.

When selector 102 is. selectively displaced by pressing the lower portion thereof inwardly, the top end of the selector is moved outwardly so as not to be contactable. by lip 80 when trigger 68 is. pulled upwardly. Thus, as hereinafter explained, when the lower portion of selector 102 is selectively pressed inwardly, automatic fire at full rate is eifected and, when the upper portion of the selector is pressed inwardly, firing is at a reduced rate.

Depending from the bottom of trigger block 20. is a pair of longitudinally disposed and laterally spaced plates. 106 which are positioned so that the forward end of actuator 57 is slidingly received therebetween. A pin 10.8. is transversely disposed through plates 106 and such pin supports a secondary sear for pivotal displacement between a bolt holding and a bolt release position. Pin 108 extends through secondary sear 110 at the rear end thereof and provided on the underside of the sec-* ondary sear at the front end thereof is a catch 112 which, when the secondary sear is in the bolt holding position, is in engagement with a secondary locking lug 114, provided on the top of bolt 14 forwardly of locking lug 44, to interrupt counterrecoil movement of the bolt.

Provided along the top of secondary sear 110 is a planar surface 116 which extends rearwardly from the. front end thereof to a hook 118 which is disposed above pin 108 and slightly rearward of the axis thereof. Hook 118 is arranged so as to be received by recess 59 in actuator 57 and engaged by wall 60 therein to pivot secondary sear 110 to the bolt release position when sear block 22 is normally disposed relative to trigger block 20. During the forward displacement of scar block 22 relative to trigger block 20, front end 61 of actuator 57 moves along planar surface 116 to press the front end of secondary sear 110 downwardly to the bolt holding position and hold catch 112 in engagement with secondary locking lug 114. The upward displacement of the front end of secondary sear 110 is limited by the contact thereof with a pin 120 mounted transversely through plates 106.

Thus, when firearm 12 is to be operated automatically at a reduced rate of fire, the top portion of selector 102 is pushed in to swing the top end thereof into position to contact lip 80 as shown in FIG. 5. Therefore, when trigger 68 is pulled upwardly to initiate a burst of fire, the downward displacement of end 66 of trigger bar 62 carries with it trigger release 74 and thereby leg 78 is actuated against plunger 86. This moves the bottom end of plunger 86 against planar surface 55 on forward end 54 of lever 52 (FIGS. 1 and 2) which is pivoted thereby to elevate sear 40 to the bolt release position. Whereby, latch portion 42 of sear 40 is disconnected from locking lug 44 to release bolt 14 for forward travel to the battery position under the compulsion of spring 18 for firing the chambered round (not shown).

When trigger release 74 was moved downwardly with trigger bar 62, lip 80, as shown in FIG. 5, was displaced upwardly by the contact thereof with the top of selector 102' thereby rotating the trigger release relative to the trigger bar. The rotation of trigger release 74 swings the end of leg 78 across the top of plunger 86 so as to be clear thereof immediately after the plunger is displaced sufficiently to actuate sear 40 to the bolt release position. When leg 78 is clear thereof, plunger 86 returns to its normal position through the bias of its cooperating spring, permitting sear 40 to return to its normal bolt engaging position. When trigger 68 was pulled upwardly, nub 97 on end 66 thereof was moved downwardly against the top of arm 90 of disconnector 88 to swing leg 92 to its vertical position in front of and in alignment with forward end 54 of lever 52 (FIG. 6). i

As bolt 14 moves in recoil flight responsive to the discharge of the chambered cartridge (not shown), sear 40 is pushed upwardly out of the way of locking lug 44 on a the bolt through the engagement of the ramps 46 and 47. After locking lug 44 passes by sear 40, the sear snaps down in front thereof and is ready to engage the locking lug after bolt 14 contacts buffer device 16 and starts its counterrecoil flight.

When locking lug 44 contacts latch portion 42 of sear 40 during counterrecoil flight of bolt 14, the energy in the bolt is transferred through the sear to sear block 22. The transferred energy causes sear block 22 to move forwardly against the bias of spring 38 which is compressed in cylinder 24 as piston 32 is held stationary by the contact thereof with trigger block 20 and the cylinder slides over the piston to move nut 39 forwardly against the rear end of the spring. As sear block 22 is displaced forwardly relative to trigger block 20, lever 52 carried by the sear block is moved against leg 92 of disconnector 88 mounted in trigger block 20. Whereby, as shown in FIG. 4, cam surface 98 on leg 92 engages cam surface 100 on forward end 54 of lever 52 to pivot such forward end downwardly and thereby elevate sear 40 to release bolt 14' for forward travel through the bias of spring 18.

However, during the forward displacement of sear block 22 relative to trigger block 20 when sear 40 engaged bolt 14, actuator 57 was displaced forwardly with the sear block to move front end 61 of the actuator along planar surface 116 of secondary sear 1'10. Whereby, hook 118 was moved into position to engage secondary locking lug 114 after bolt 14 was released by sear 40 and thereby interrupt the forward travel of the bolt.

When sear 40 was released from bolt 14, sear block 22 was freed to be returned rearwardly to its normal position by spring 38. This rearward displacement of sear block 22 relative to trigger block 20 moves actuator 57 rearwardly also and, when the sear block approaches its normal position, wall 60 on the actuator engages catch 112 on secondary sear 110 (FIG. 3) which is rotated thereby to elevate hook 118 to free bolt 14 for free travel to battery position. To end the firing burst, the pull on trigger 68 is relieved permitting trigger bar 62 to be pivoted to normal position by spring 70. The elevation of end 66 of trigger bar 62 frees disconnector 88 so as to permit pivotal displacement thereof by plunger 96 to its normal position whereby leg 92 is swung out of alignment with lever 52. Therefore, when the bolt is engaged by sear 40 during the next counterrecoil travel of the bolt after trigger 68 is released, the cam surfaces 98 and 100 cannot engage to pivot lever 52 for actuating sear 40 to the bolt release position. Consequently, bolt 14 is releasably held by sear 40 until trigger 68 is pulled to initiate another burst. The time delay introduced into the operating cycle of firearm 12 is determinable by the tension of spring 38 and the predetermined distancex -x which sear block 22 is displaceable relative to trigger block 20 so the desired reduced rate of fire may be achieved by variations of these determining factors.

When firearm 12 is to be fired automatically at full rate, the bottom portion of selector 102 is pressed in (FIG. 2) to move the top end thereof clear of lip so that, when trigger bar 62 is pivoted, trigger release 74 will not be pivoted relative thereto. Therefore, when trigger 68 is pulled upwardly to initiate a burst, trigger release 74 is pressed against plunger 86 to pivot lever 52 for elevating sear 40 to the bolt release position and the connection between trigger bar 62 and lever 52 is maintained to hold the sear in the bolt release position until the pull on the trigger is relieved.

From the foregoing it is clearly apparent that there is provided herein a dual rate sear mechanism wherein both the primary sear and the secondary sear are positively held in the bolt engaging positions thereof to eliminate the hazard of having the firearm run away due to improper searing. It is also evident that the sear mechanism is simple in design, positive in operation and easy to manufacture.

Although a particular embodiment of the invention has been described in detail herein, it is evident that many variations may be devised within the spirit and scope thereof and the following claims are intended to include such variations.

I claim:

1. A sear mechanism for a firearm provided with a receiver having a bolt disposed therein for reciprocation between a recoil and a battery position and arranged to be fired from the open bolt position, the sear mechanism including a trigger block secured to the receiver, a sear block mounted on said trigger block for longitudinal reciprocation respective thereto a predetermined distance between a forward and rearward position, a primary sear carried by said sear block for releasable engagement with a primary locking lug on the bolt to releasably hold the bolt adjacent the recoil position, a secondary sear carried by said trigger block for releasable engagement with a secondary locking lug on the bolt after release by said primary sear for displacement to the battery position, spring-biased means arranged for resiliently biasing said sear block to the rearward position thereof respective to said trigger block and for transferring the energy in said bolt to said sear block for reciprocal displacement thereof the predetermined distance when the bolt is engaged by said primary sear, a trigger bar pivotally mounted on said trigger block, means for transferring pivotal displacement of said trigger bar to actuation of said primary sear for release of the bolt, and mechanical means arranged to selectively convert the displacement of said sear block relative to said trigger block to actuation of said primary sear for release of the bolt and sequential actuation of said secondary sear into position for engagement with said secondary locking lug to delay the travel of the bolt to the battery position a period of time determinable by the tension of said spring-biased means and the predetermined distance of reciprocation of said sear block relative to said trigger block.

2. The sear mechanism as defined in claim 1 wherein said spring-biased means include a cylinder integral with said sear block, a piston slidingly mounted in said cylinder, a coil spring disposed in said cylinder rearwardly of said piston, a stem extendingfrom said piston for contact with said trigger block to move said piston against said spring during forward displacement of said sear blockrelative to said trigger block, and a nut threadingly mounted in the rear end of said cylinder for contact with the adjacent end of said spring whereby when said primary sear engages the bolt the energy in the bolt during counterrecoil travel thereof is absorbed by said spring and transferred therefrom to said sear block for returning said sear block to the rearward position thereof relative to said trigger block.

3. The scar mechanism as defined in claim 1 wherein said secondary sear is pivotally mounted on said trigger block and includes a catch disposed for releasable engagement with said secondary locking lug and wherein said mechanical means includes an actuator mounted on .said sear block for engageable cooperation with said secondary sear whereby said secondary sear is pivoted by said actuator to a position for engaging said catch with said secondary locking lug during forward displacement of said sear block relative to said trigger block and to a position for disengaging said catch from said secondary locking lug during rearward displacement of said sear block relative to said trigger block.

4. The sear mechanism as defined in claim 1 wherein said secondary sear is pivotally mounted at the rear end thereof on said trigger block and includes a planar surface extending along the top thereof, a hook formed at the rear end of said planar surface and a catch disposed at the front end for releasable engagement with said secondary locking lug, and wherein said mechanical means include a cylindrical actuator carried by said sear block for longitudinal extension therefrom into engageable cooperation with said secondary sear, saidactuator being provided with a recess, a wall and a front end formed by the recess, said front end being disposed for sliding engagement with said planar surface for pressing said 8 secondary ear into position for engagement of said catch with said secondary locking lug during forward displacement of said sear block relative to said trigger block, and

said wall being disposed for contact with said hook to pivot said secondary sear for disengaging said catch from. said secondary locking lug during rearward displacement of said sear block relative to said trigger block.

' 5. The sear mechanism as defined in claim 1 wherein said primary sear is slidingly mounted in said sear block for displacement normal to the travel of the bolt and said means for transferring pivotal displacement of said trigger barto said primary sear for release of thebolt includes a lever pivotally mounted in said sear block, said lever being provided with one end engaged to said primary sear for transferring pivotal displacement of said lever thereto and a second end extending forwardly into saidtrigg'er block, a plunger slidingly disposed in said trigger block for action against said second end to pivot said I pivotal displacement to a position clear of said plunger and including a selector mounted on said trigger block so as to be selectively positioned for contact with said trigger release during pivotal displacement of said trigger bar to a firingtposition to transfer pivotal displacement of said trigger bar to pivotal displacement of said trigger release out of contact with said plunger after said lever is pivoted thereby to actuate said primary sear to the bolt release position.

No references cited. 

